Solvent stripping process

ABSTRACT

A process for stripping spinning solvent from a solution-spun nonwoven web by transporting a nonwoven web comprising solvent-laden polymeric fibers having average fiber diameters of less than about 1 micrometer through a solvent stripping zone wherein a solvent stripping fluid heated to at least about 70° C. impinges on the nonwoven web in order to reduce the solvent concentration of the fibers to less than about 10,000 ppmw.

A process for stripping solvent from solvent-laden fibers in asolution-spun fiber web is disclosed.

BACKGROUND

The process of solution spinning involves dissolving a desired polymerinto a suitable solvent, and spinning fibers from the polymer/solventsolution. Often, the solvent is an organic solvent which has undesirableproperties in use of the so-formed fabric, such as adverse healtheffects, undesired odor and the like. It would be desirable to strip theunwanted solvent from the fibers or fabric during the productionprocess, prior to shipping to the ultimate customer.

Solution spinning processes are frequently used to manufacture fibersand nonwoven fabrics, and in some cases have the advantage of highthroughputs, such that the fibers or fabrics can be made in large,commercially viable quantities. Unfortunately, when solution spinninglarge quantities of fabric at high throughput through the spinning dies,significant quantities of residual solvent can be entrained in thecollected fabrics or fibers. Ideally, the residual solvent would merelyevaporate upon sitting, leaving the fabric solvent-free, but in manycases the ideal solvent used for the solution spinning process has ahigh chemical or physical affinity for the fiber polymer. In some cases,the fiber polymer is swollen by the solvent; i.e. the solvent is“dissolved” within the fiber polymer. In other cases the solventchemically bonds to the fiber, such as by hydrogen bonding, Van derWaals forces, or even ionically via salt formation.

Further, in typical nonwoven fabric spinning processes, the fabric isspun and wound into a large roll in an essentially continuous operation,such that even if the solvent were amenable to evaporation upon sitting,only the solvent entrained in the fabric on the outside of the roll iseffectively evaporated, since the underlying fabric within the roll isnot exposed to the atmosphere. Detrimentally, even if the fabric were tobe provided sufficient time in the unrolled state to permit the spinningsolvent to evaporate, an exceedingly long area would be necessary toprovide room for the unrolled fabric, and recovery of the evaporatedsolvent would be difficult and expensive.

In paper making processes, such as those disclosed in U.S. Pat. Nos.3,503,134 and 6,986,830, dewatering of the wet laid cellulose fiberswhich form the paper is performed by passing the wet laid cellulose webover a vacuum-assisted porous drum, and the excess water from theforming process is drawn through and away from the paper web. U.S. Pat.No. 3,503,134 discloses the use of hot air, superheated steam or asteam-air mix to enhance the drying effect of the vacuum assist. U.S.Pat. No. 6,986,830 discloses positioning the wet laid paper web betweentwo soft, porous cloth webs, wherein the porous cloths on either side ofthe paper web pull additional water from the paper by capillary action.However, in either case, while it is advantageous to remove as muchwater as possible from the wet laid paper web, residual water isnon-toxic and would not cause adverse health effects if present in thefinished product.

U.S. Published Patent Application No. 2002/0092423 discloses a solutionspinning process for forming a nonwoven polymer web, in particular anelectrospinning process, wherein polymeric microfibers or nanofibers areproduced from a polymer solution exiting an electrically-chargedrotating emitter and directed toward a grounded collector grid. However,according to the applicants thereof, the solvent is evaporated from thefibers “in flight” between the emitter and the collector grid. Thethroughput of the electrospinning process disclosed in U.S. PublishedPatent Application No. 2002/0092423 is relatively low at about 1.5ml/min/emitter, and as such would form relatively light basis weightpolymer webs.

SUMMARY OF THE INVENTION

In a first embodiment, the invention is a process for strippingchemically bonded spinning solvent from a solution-spun nonwoven webcomprising the steps of providing a nonwoven web comprisingsolvent-laden polymeric fibers having average fiber diameters of lessthan about 1 micrometer, and transporting the nonwoven web through asolvent stripping zone wherein a solvent stripping fluid heated to atleast about 70° C. impinges on the nonwoven web in order to reduce thesolvent concentration of the fibers to less than about 10,000 ppmw.

In another embodiment, the invention is a solution-spun nonwoven webcomprising polymeric fibers having average fiber diameters of less thanabout 1 micrometer and containing less than about 10,000 ppmw ofspinning solvent which bonds with the fiber polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate the presently contemplatedembodiments of the invention and, together with the description, serveto explain the principles of the invention.

FIG. 1 is a schematic of a prior art nanofiber web preparing apparatusfor preparing a filtration medium according to the invention.

FIG. 2 is a schematic of a solvent stripping station according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to solvent-spun webs and fabrics for avariety of customer end-use applications, such as filtration media,protective apparel and the like, including at least one nanofiber layer,and a process for removing excess spinning solvent from thesolution-spun nanofiber webs or fabrics.

There is a need for fibrous products made from a wide variety ofpolymers to suit various customer end-use needs. Many polymeric fibersand webs can be formed from melt spinning processes, such as spunbonding and melt blowing. However, the ability to use melt spinning islimited to spinning fibers from polymers which are melt processable,i.e. those which can be softened or melted and flow at elevatedtemperatures. Still, in many end-uses, it is desirable to utilizepolymers which are not melt processable, for example thermosettingpolymers and the like, to form fibrous materials, fabrics and webs. Inorder to form these non-melt-processable polymers into fibrousmaterials, the technique of solution spinning is used.

As discussed above, solution spinning processes, such as wet spinning,dry spinning, flash spinning, electrospinning and electroblowing,involve dissolving a desired polymer into a suitable solvent, andspinning fibers from the polymer/solvent solution. Often, the solvent isan organic solvent which has undesirable properties in use of theso-formed fabric, such as adverse health effects, undesired odor and thelike. It would be desirable to strip the unwanted solvent from thefibers or fabric during the production process, prior to shipping to theultimate customer.

Unfortunately, when solution spinning large quantities of fabric at highthroughput through the spinning dies, such as to form nonwoven webshaving basis weights of greater than about 5 grams/square meter (gsm),significant quantities of residual solvent can be entrained in thecollected fabrics or fibers, due to either or both of high physical orchemical affinities of the solvent for the polymer so spun, and the lackof sufficient time or space between fiber formation and fiber collectionfor complete evaporation of the spinning solvent. In many cases, thesolvents used in the solution spinning processes demonstrate variouslevels of toxicity, or present negative environmental effects or causeadverse chemical reactions in particular end-uses. As such, it ispreferred to remove as much residual solvent from the solution spunfibrous materials as possible.

Solvent removal is often complicated by the fact that any particularpolymer/solvent spinning system is chosen based upon a strong affinityof the solvent for the polymer, in order to effect complete dissolutionof the polymer in the solvent during the spinning operation. In somecases, the fiber polymer is swollen by the solvent; i.e. the solvent is“dissolved” within the fiber polymer. In other cases the solventchemically bonds to the fiber, such as by hydrogen bonding, Van derWaals forces, or even ionically via salt formation.

In some prior art solvent spinning processes, such as dry spinning,removal of high affinity solvents is accomplished by spinning the fibersinto a hot gas “chimney” of as much as 30 feet in length, and passinghigh temperature gas (as high as 500° C.) through the chimney to driveoff the unwanted solvent. As can be imagined, this process involves anexpensive apparatus and is an energy-intensive process.

The present inventor has discovered that one manner of enhancingunwanted solvent removal from solution spun fibers is to reduce thediameter of the fibers themselves, since the diffusion de-volatilizationmechanisms follow a 1/diameter² relationship. That is, entrained solventwill diffuse more readily out of fibers having smaller diameters thanout of fibers having larger diameters. According to the presentinvention, it is preferred that solution spun fibers have diameters lessthan about 1 micrometer (nanofibers) to optimize the diffusionde-volatilization mechanism of solvent removal.

The term “nanofibers” refers to fibers having diameters varying from afew tens of nanometers up to several hundred nanometers, but generallyless than about one micrometer, even less than about 0.8 micrometer, andeven less than about 0.5 micrometer.

The solution spun fabrics and webs of the present invention include atleast one layer of polymeric nanofibers. The nanofibers have averagefiber diameters of less than about 1 μm, preferably between about 0.1 μmand about 1 μm, and high enough basis weights to satisfy a variety ofcommercial end-uses, such as for air/liquid filtration media, batteryseparator fabrics, protective apparel and the like.

The process for making commercial quantities and basis weights ofnanofiber layer(s) is disclosed in International Publication NumberWO2003/080905 (U.S. Ser. No. 10/822,325), which is hereby incorporatedby reference. FIG. 1 is a schematic diagram of an electroblowingapparatus useful for carrying out the process of the present inventionusing electroblowing (or “electro-blown spinning”) as described inInternational Publication Number WO2003/080905. This prior artelectroblowing method comprises feeding a solution of a polymer in asolvent from mixing chamber 100, through a spinning beam 102, to aspinning nozzle 104 to which a high voltage is applied, while compressedgas is directed toward the polymer solution in a blowing gas stream 106as it exits the nozzle to form nanofibers, and collecting the nanofibersinto a web on a grounded collector 110 under vacuum created by vacuumchamber 114 and blower 112.

The moving collection apparatus is preferably a moving collection beltpositioned within the electrostatic field between the spinning beam 102and the collector 110. After being collected, the nanofiber layer isdirected to and wound onto a wind-up roll on the downstream side of thespinning beam. Optionally, the nanofiber web can be deposited onto anyof a variety of porous scrim materials arranged on the moving collectionbelt 110, such as spunbonded nonwovens, meltblown nonwovens, needlepunched nonwovens, woven fabrics, knit fabrics, apertured films, paperand combinations thereof.

Due to the high throughput of the electroblowing apparatus, typicallybetween about 0.1 to 5 mL/hole/min, and the large number of spinningnozzles (holes) 104 distributed across the spinning beam 102, a singlenanofiber layer having a basis weight of between about 2 g/m² and about100 g/m², even between about 10 g/m² and about 90 g/m², and even betweenabout 20 g/m² and about 70 g/m², as measured on a dry basis, i.e., afterthe residual solvent has evaporated or been removed, can be made bydepositing nanofibers from a single spinning beam in a single pass ofthe moving collection apparatus. However, also due to the highthroughput of the process and the speed at which the electroblown fibersare collected on the collection belt, significant quantities of residualspinning solvent, especially those solvents with strong affinities forthe fiber polymers, can remain in the nanofiber webs so-formed.

It has been discovered that reducing fiber diameter, even to below 1micrometer, or even to below about 0.8 micrometer, or even below about0.5 micrometer, is alone insufficient to reduce or eliminate residualsolvent from the nanofiber web merely by vacuum-assisted collection.

Accordingly, the solvent stripping process and apparatus of the presentinvention, FIG. 2, which is disposed downstream of the collection belt110 of the prior art apparatus (FIG. 1), acts to effect reduction orelimination of unwanted residual solvent from solution spinningprocesses in a continuous manner, prior to wind-up of the fabric or web.

The solvent stripping apparatus comprises a continuous moving belt 14for supporting the solvent spun nanofiber web and its optionalsupporting scrim 10 and directing it through one or more solventstripping stations 20, each of which comprise a fresh solvent strippingfluid heating apparatus 16, disposed on one side of the moving belt 14,and a vacuum apparatus 18, disposed on the opposite side of moving belt14. The “fresh solvent stripping fluid” 17, typically air, is impingedupon the moving solution spun web, and the vacuum apparatus helps todraw the stripping fluid through the solution spun web to effect solventstripping. Preferably, a spent solvent stripping fluid collector (notshown) is disposed downstream of the vacuum apparatus to scrub theexcess spinning solvent from the spent stripping fluid for recycling ordisposal.

The fresh solvent stripping fluid can be a gas selected from air,nitrogen, argon, helium, carbon dioxide, hydrocarbons, halocarbons,halohydrocarbons, and mixtures thereof, and is essentially free fromvapors of the spinning solvent to be stripped, such that the partialpressure of the spinning solvent is much higher within the polymerfibers of the solution spun web than in the solvent stripping fluid, soas to drive diffusion of the residual stripping solvent from thesolvent-laden polymer fibers into the solvent stripping fluid. However,even this differential in partial pressures is insufficient to extract aspinning solvent with high affinity for the fiber polymer down toconcentration levels on or within the fibers which are suitable for manyconsumer uses.

It has been discovered that in order to reduce the concentration levelof solvents with strong affinities for the fiber polymer to less than 1wt % (10,000 ppmw) in a continuous process, it is necessary to heat thefresh solvent stripping fluid to temperatures of at least about 70° C.,or at least about 90° C., or at least about 110° C. and even at leastabout 150° C., up to as high as the melting point of the polymer (in thecase of a thermoplastic polymer) or just below the decompositiontemperature of the polymer (in the case of a non-thermoplastic polymer)for short periods of time to avoid polymer melting or decomposition.

The present inventor deems the relatively high temperatures necessary tode-couple the spinning solvents from the polymers to be unexpected, asthe skilled artisan would expect that the solvent would evaporate atroom temperatures within the space between the spinning nozzles and thecollector, as set forth in U.S. Published Patent Application No.2002/0092423. Instead, it was found to be necessary to applytemperatures well-above the spinning solvent boiling point to reduce thespinning solvent levels to less than about 1000 ppmw in a continuousprocess and within a commercially viable time (see Examples, below).

Utilizing the combination of “fresh” solvent stripping fluid (i.e. onehaving very low partial pressure of the spinning solvent) and increasedtemperature of the solvent stripping fluid, it is possible to reduce thesolvent concentration on or in the fiber polymer to less than about10,000 ppmw, even to less than 1000 ppmw, or even less than about 300ppmw.

Depending, of course, on the affinity of the particular spinning solventfor the fiber polymer, it may be advantageous to incorporate more thanone solvent stripping station into the solvent stripping apparatus, soas to reduce the residual solvent concentration in multiple steps. Thetemperature, vacuum pressure and even the fresh solvent stripping fluiditself can be individually controlled within each solvent strippingstation.

Polymer/solvent combinations which can benefit from the presentinvention are those in which the polymer exhibits a strong affinity forthe solvent, particularly those in which chemical bonding occurs betweenthe polymer and the solvent, such as hydrogen bonding and the like. Somecombinations of polymer/solvent which are difficult to separate arepolyamide/formic acid and polyvinyl alcohol/water.

EXAMPLES

The examples below were prepared from a polymer solution having aconcentration of 24 wt % of nylon 6,6 polymer, Zytel® FE3218 (availablefrom E. I. du Pont de Nemours and Company, Wilmington, Del.) dissolvedin formic acid solvent at 99% purity (available from Kemira Oyj,Helsinki, Finland) that was electroblown to form a nonwoven webcontaining some residual solvent. The heavier basis weight examples werecollected at lower collection belt speeds, and the solvent strippingparameters were varied as set forth in the Table.

The residual formic acid content in the nonwoven sheets of nylon wasdetermined using standard wet chemistry techniques and ionchromatography analysis. In a typical determination, a sample of knownmass was placed in caustic solution. An aliquot of the resultingsolution was analyzed by ion chromatography and the area under the peakcorresponding to neutralized formic acid (formate anion) wasproportional to the quantity of formic acid in the sample.

Comparative Example 1 Control

Example 1 was prepared as set forth above, but was not subjected to thesolvent stripping process of the present invention. The initial level ofsolvent upon web laydown was 46.2 wt % of the nonwoven web.

Comparative Example 2

Comparative Example 2 was prepared in the manner of Comparative Example1, except rather than collecting and analyzing the nonwoven sheetdirectly after laydown, the nonwoven web was transported into a solventstripping zone on a moving porous screen. A solvent stripping fluid ofair at a temperature of 25° C. was impinged onto the nonwoven web fromone side while a vacuum was applied to the other side of the nonwovenweb. The vacuum was measured at 58 mm H₂O. The air pressure and thevacuum were coupled to yield a near constant atmospheric pressure in thesolvent stripping zone. The nonwoven web remained in the solventstripping zone for 9.5 seconds. The final basis weight of the nonwovenweb was 5.2 g/m². The final solvent level was 1.0 wt % of the nonwovenweb.

Comparative Examples 3-4

Comparative Examples 3-4 were prepared in the same manner as ComparativeExample 2, except a slower collection belt speed was employed, resultingin higher basis weights and longer residence times in the solventstripping zone. At these higher basis weights it was necessary to employhigher vacuum levels to maintain solvent stripper fluid flow through theweb. A consequence of the solvent stripping zone time increase was theremoval of additional solvent (on a weight percentage basis) from thehigher basis weight nonwoven webs. A cooling zone was not used. Thesedata and the final solvent level are summarized in the Table.

Examples 1-4

Examples 1-4 were prepared in the same manner as Comparative Examples2-4, except an elevated solvent stripping zone temperature was used.These data and the final solvent level are summarized in the Table.

TABLE Solvent Stripping Efficiency Temperature, Vacuum, ResidenceMeasured Measured ° C. mm H₂O Time, sec Basis Weight, Solvent Conc,Example (stripping zone) (stripping zone) (stripping zone) g/m² wt %(ppm) CE 1 NA NA NA 5  46.2 (462,000) CE 2 25 58 9.5 5.2  1.0 (10,000)CE 3 25 96 38.1 19.9  2.5 (25,000) CE 4 25 116 75.9 40.4  2.2 (22,000) 190 56 9.5 5.4 0.8 (8,000) 2 90 100 38.1 22.9 0.5 (5,000) 3 90 116 75.945.8 0.3 (3,000) 4 150 * 38.3 30.0 0.03 (300)   *Vacuum Transducer notoperational, but vacuum was applied. Air was forced through the sheet tomaintain low formic acid partial pressure in vicinity of fibers.

Comparative Example 1 demonstrates the level of stripping solvent whichis entrained in the nanofiber webs after the prior art electroblowingprocess.

Comparative Examples 2-4 show that residual formic acid levels are veryhigh at low stripping temperatures. Even in these cases, the appliedvacuum provided convective flow which maintained the low solventactivity levels in vicinity of the fibers, but not enough solvent wasremoved from the webs to satisfy most commercial applications. Theseexamples also show the effect of residence time.

Examples 1-3 show the effect of elevated stripping temperatures, whichremoved more residual solvent to levels suitable for some commercialuses.

Example 4 shows that a stripping temperature well in excess of theboiling point of the solvent (101° C. for formic acid) results inextremely low residual solvent level in the electrospun web.

These examples demonstrate that the solvent stripping zone of thepresent invention can prepare a solution spun nonwoven web that issubstantially free of spinning solvent. Review of the data from theTable show that increasing the solvent stripping zone temperature,vacuum and residence time, improves the efficiency of solvent removalfrom the nonwoven webs.

1. A process for stripping chemically bonded spinning solvent from asolution-spun nonwoven web comprising the steps of: providing a nonwovenweb comprising solvent-laden polymeric fibers having average fiberdiameters of less than about 1 micrometer, and transporting the nonwovenweb through a solvent stripping zone wherein a solvent stripping fluidheated to at least about 70° C. impinges on the nonwoven web in order toreduce the solvent concentration of the fibers to less than about 10,000ppmw.
 2. The process according to claim 1 wherein the average fiberdiameter is less than 0.8 micrometer.
 3. The process according to claim1, wherein the solvent stripping fluid is heated to between about 70° C.and the melting point of the fiber polymer.
 4. The process according toclaim 1, wherein the solvent stripping fluid is heated to between about70° C. and the decomposition point of the fiber polymer.
 5. The processaccording to claim 1, wherein the solvent stripping fluid is selectedfrom the group of air, nitrogen, argon, helium, carbon dioxide,hydrocarbons, halocarbons, halohydrocarbons, and mixtures thereof. 6.The process according to claim 5, wherein the solvent stripping fluid isair.
 7. The process according to claim 2, wherein the average fiberdiameter is less than 0.5 micrometer.
 8. The process according to claim1, wherein the solvent concentration is reduced to less than 1,000 ppmw.9. The process according to claim 8, wherein the solvent concentrationis reduced to less than 300 ppmw.
 10. The process according to claim 1,further comprising transporting the web through the solvent strippingzone by pinning the nonwoven web to a moving porous belt with a vacuumsource located on the side of the porous belt opposite the nonwoven web,and passing fresh solvent stripping fluid through the web.
 11. Theprocess according to claim 1, wherein the nonwoven web is transportedthrough the solvent stripping zone on top of a scrim.
 12. The processaccording to claim 1, further comprising passing the nonwoven webthrough at least one additional solvent stripping zone.
 13. Asolution-spun nonwoven web comprising polymeric fibers having averagefiber diameters of less than about 1 micrometer and containing less thanabout 10,000 ppmw of spinning solvent which bonds with the fiberpolymer.
 14. The solution-spun nonwoven web of claim 13, wherein saidfibers contain less than about 1000 ppmw of said spinning solvent. 15.The solution-spun nonwoven web of claim 12, wherein said fibers containless than about 300 ppmw of said spinning solvent.
 16. The solution-spunnonwoven web of claim 13, wherein said polymeric fibers have averagefiber diameters of less than 0.5 micrometer.